Intake system for a two-stroke internal combustion engine

ABSTRACT

A combustion chamber of an internal combustion engine has primary intake ports located a first distance from the crankshaft and primary intake ports located a second distance from the crankshaft. One or more unidirectional valves, such as reed valves, are place in an intake duct upstream of the primary intake ports. The valves prevent backflow from the cylinder into the intake when the pressure in the cylinder exceeds that of the intake. The backflow into the intake is reduced by having valves in the intake; therefore scavenging efficiency is improved. Furthermore, the asymmetry between the intake and exhaust port timing may be reduced by providing such valves. Additionally, the total intake port area can be increased with two sets of intake ports and valves disposed in the primary ports, thereby improving the volumetric efficiency in spite of the pressure drop presented by the valves.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority benefit from U.S. provisional patent application 61/478,736 filed 25 Apr. 2011.

FIELD

The present disclosure is related to internal combustion engines having primary and secondary intake ports. The present disclosure is further related to engines having a unidirectional valve disposed in the primary intake ports.

BACKGROUND

Two-stroke engines are commonly provided intake and exhaust ports in the cylinder wall through which intake gases are provided into the cylinder and exhaust gases are scavenged from the cylinder. The ports are covered and uncovered by pistons reciprocating in the cylinder and the open durations of the intake and exhaust ports overlap. During overlap and at some operating conditions, the respective pressures in the cylinder and the intake system are such that exhaust gases from the cylinder backflow through the intake ports into the intake system thereby negatively impacting scavenging effectiveness of the engine. It is known to provide reed valves in the intake system as close to the intake ports as possible to largely prevent backflow of exhaust gases into the intake system. Reed valves allow flow from the intake system into the cylinder; however, the valves close off flow when pressure in the cylinder is greater than the pressure in the intake, thereby preventing back flow into the intake. Reed valves, unfortunately, present a pressure drop in the intake system and thus reduce the peak torque that the engine may develop. It is desirable to improve scavenging effectiveness, but without increasing pressure drop in the intake.

It is desired to provide a larger intake port area than exhaust port area because the driving pressure on the exhaust gases is greater than the driving pressure on the intake gases. In an engine in which the ports are provided substantially evenly around the periphery, the total intake port height is greater than that of the exhaust port height to provide the desired flow area. However, such greater intake port height is inconsistent with the desired port timing in which the exhaust ports open and close prior to the opening and closing of the intake ports.

SUMMARY

To address at least one issue associated describe above, an internal combustion engine is disclosed which has a cylinder, a crankshaft, a piston reciprocating in the cylinder with the piston coupled to the crankshaft via a rod, a first plurality of intake ports defined in the cylinder at a first predetermined distance from an axis of rotation of the crankshaft, and a second plurality of intake ports defined in the cylinder at a second predetermined distance from an axis of rotation of the crankshaft. The first and second pluralities of ports fluidly couple between a combustion chamber defined by a top of the piston and the cylinder In some embodiments, the first and second pluralities of ports are substantially evenly disposed around a periphery of the cylinder. Normally-closed unidirectional flow valves are disposed upstream of the first plurality of intake ports. The unidirectional valves open when pressure on the upstream side of the unidirectional valves exceeds the pressure on the downstream side of the unidirectional valves. In some embodiments, the unidirectional valves include a reed valve block having a frame with a plurality of openings, flexible petals adapted to cover the openings, and stops that limit the movement of the petals. The first plurality of intake ports do not overlap the second plurality of intake portions in a direction parallel to motion of the piston in the cylinder. In some situations, a high-pressure compressor is fluidly coupled to the first plurality of intake ports and a low-pressure compressor is fluidly coupled to the second plurality of intake ports. The second plurality of intake ports is closer to the crankshaft than the first plurality of intake ports.

In some examples, the piston is an intake piston and the engine further includes: a first plurality of exhaust ports defined in the cylinder at a third predetermined distance from the axis of rotation of the crankshaft, and an exhaust piston reciprocating in the cylinder with a top of the intake piston facing a top of the exhaust piston in an opposed-piston arrangement wherein the intake piston covers and uncovers the first and second pluralities of intake ports when the intake piston reciprocates in the cylinder and the exhaust piston covers and uncovers the first plurality of exhaust ports when the exhaust piston reciprocates in the cylinder. The engine may further include a second plurality of exhaust ports defined in the cylinder at a fourth predetermined distance from the axis of rotation of the crankshaft, a high-pressure compressor coupled to the first plurality of intake ports, a low-pressure compressor coupled to the second plurality of intake ports, a high-pressure turbine fluidly coupled to the first plurality of exhaust ports, and a low-pressure turbine fluidly coupled to the second plurality of exhaust ports wherein the high-pressure compressor is shaft-coupled to the high-pressure turbine and the low-pressure compressor is shaft-coupled to the low pressure turbine.

Also disclosed is an internal combustion engine having a cylinder, a crankshaft, a piston adapted to reciprocate in the cylinder with the piston coupled to the crankshaft via a rod, a plurality of primary intake ports defined in the cylinder at a first predetermined distance from an axis of rotation of the crankshaft, a plurality of secondary intake ports defined in the cylinder at a second predetermined distance from the axis of rotation of the crankshaft, a primary intake duct coupled to the primary intake ports, normally-closed valves disposed in the primary intake duct, and a secondary intake duct coupled to the secondary intake ports. In some embodiments, reed valves are disposed in the secondary intake duct. The reed valves may be disposed in a reed valve block. The primary intake duct is fluidly decoupled from the secondary intake duct at least for a portion of the lengths of the first and secondary intake on an end of the intake ducts proximate the intake ports.

Also disclosed in an internal combustion engine having a cylinder with a plurality of primary intake ports defined in the cylinder at first predetermined distance from a first end of the cylinder and a plurality of secondary intake ports defined in the cylinder at a second predetermined distance from the first end of the cylinder, a primary intake duct coupled to the primary intake ports, and a secondary intake duct coupled to the secondary intake ports. The primary intake duct is fluidly decoupled from the secondary intake duct at least for a portion of the lengths of the first and secondary intake ducts proximate the intake ports. The engine may also include an intake plenum fluidly coupled to the primary and secondary intake ducts and a unidirectional valve disposed in the primary intake duct located between the intake plenum and the primary intake ports. The engine further includes a reed valve block disposed in the primary intake duct. The reed valve block includes a frame having a plurality of openings and flexible reed petals adapted to occlude the openings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an opposed-piston engine;

FIG. 2 is a cross-section of a vee-configured, piston-ported engine;

FIGS. 3, 5, and 7 are cross-sectional representations of an opposed-piston engine in which the pistons are shown in a range of positions;

FIGS. 4, 6, and 8 show details of a portion of FIGS. 3, 5, and 7, respectively;

FIG. 9 is a cylinder block for an opposed-piston engine shown in a perspective view;

FIG. 10 is a holder for a reed valve that coupled to a flange of the block shown in FIG. 9;

FIGS. 11 and 12 are cross sectional views of the cylinder block of FIG. 9, but also including the reed valve holder and intake ducts; and

FIG. 13 is a plot of port open area for an embodiment with primary and secondary intake ports.

DETAILED DESCRIPTION

As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations. Those of ordinary skill in the art may recognize similar applications or implementations whether or not explicitly described or illustrated.

In FIG. 1, an opposed-piston, piston-ported engine 80 is represented in cross section. Exhaust piston 82 and intake piston 84 reciprocate within cylinder 86. Piston 82 is coupled to a connecting rod 90 via wrist pin 88 with rod 90 coupled to a crankshaft 92. Similar components, 90′, 88′, and 92′ are provided for piston 84 as well. Pistons 82 and 84 are shown near or at bottom dead center (BDC) position, i.e., a position in which the volume contained within the cylinder and between the two piston tops is at or near its maximum. In such a position, a first plurality of intake ports 94, a second plurality of intake ports 95, a first plurality of exhaust ports 96, and a second plurality of exhaust ports 98 are uncovered by the associated pistons. The intake and exhaust routings from the first and second pluralities of intake and exhaust ports 94, 95, 96, and 98 are not illustrated explicitly in FIG. 1.

A vee-configured engine 100 is shown in FIG. 2. An intake manifold 102 feeds fresh air to both banks of cylinders 103 with fresh air provided to cylinders 103 through primary intake ports 104 and secondary intake ports 106 into combustion chambers 108. Exhaust is expelled through primary exhaust ports 110 and secondary exhaust ports 112 into exhaust ducts 114. Cylinders 103 have cylinder heads 116 with fuel injectors 118 disposed therein. In spark ignition embodiments, a spark plug is also provided in cylinder heads 116. In the left bank cylinder, a piston 120 is shown. No piston is shown in the right bank cylinder so that the ports can be seen in FIG. 2. Piston 120 is connected to a crankshaft 122 via a connecting rod (not shown in FIG. 2). At the lower end of engine 100 is a crankcase 124.

Crankshafts 92 and 92′ of FIG. 1 may be timed so that exhaust piston 82 uncovers exhaust ports 96 prior to intake piston 84 uncovering intake ports 94 during expansion. During compression, exhaust ports 96 are covered by exhaust piston 82 prior to intake ports 94 being covered by intake piston 84. Such asymmetry in the movement of the pistons leads to imbalance. The more that the movement of the pistons mirror each other, the less the imbalance. It has been found through modeling that about a 20-degree offset provides an appropriate amount of asymmetry to the port timing to provide acceptable scavenging over the range of operating conditions. It is desirable, however, to reduce the degree of asymmetry in the piston movement.

Referring now to FIG. 3 and to FIG. 4, the latter of which shows a detail of the area enclosed by the dotted line in FIG. 3, a cross section of a portion of an opposed-piston engine 3150 is shown. A cylinder 162 has an intake piston 152 and an exhaust piston 154. Exhaust ports 156 are uncovered by exhaust piston 152 during an expansion stroke. At the crank angle shown in FIG. 3, the exhaust ports 156 are partially uncovered. Two rows of intake ports: primary intake ports 158 and secondary intake ports 160 are uncovered by piston 154 during expansion. At the crank angle shown in FIG. 3, only primary intake ports 158 are uncovered, while secondary intake ports 160 remain occluded by intake piston 154. Although primary intake ports 158 are uncovered, flow through intake ports 158 is prevented by reed valves provided in the intake duct. Petals 174 of the reed valve press against frame 178 sealing off flow between an intake plenum 164 and cylinder 162. In the situation illustrated in FIG. 3, the pressure in cylinder 162 exceeds that in intake plenum 164. When pistons 152 and 154 move away from each other further and the pressure in cylinder 162 reduces, petals 174 lift from frames 178 thereby allowing intake flow into cylinder 162. In the embodiment shown in FIGS. 3 and 4, reed valves are also provided in the secondary intake ports 160, with petals 176 sealing against stops in frames 180. In an alternative embodiment, reed valves are provided only in primary intake ports 158 and not in secondary intake ports 160.

In FIG. 5, engine 150 is shown at a crank angle at which pistons 152 and 154 are further apart from each other than in FIG. 3. Exhaust ports 156 are completely uncovered. The pressure in cylinder 162 is such that petals 174 lift from frames 178 and allow flow from intake plenum 164 into cylinder 162. Secondary intake ports 160 are occluded by both piston 154 as well as petals 176 of the reed valves associated with secondary intake ports 160. Detail of the area enclosed by the dashed line is shown In FIG. 6.

An even further expanded state in cylinder 162 is shown in FIGS. 7 and 8. Both sets of intake ports 158 and 160 are uncovered. As the pressure in the cylinder is sufficiently reduced, as soon as intake piston 154 uncovers secondary intake ports 160, petals 176 lift from frames 160.

In FIG. 9, an embodiment, a block 200 of an opposed-piston engine is shown in perspective. An opening 202 to the exhaust ports is provided in block 200. An opening 204 leading to primary intake ports and an opening 206 leading to secondary intake ports are shown siamesed on the outer surface of block 200.

In FIG. 10, a frame 210 has a primary opening into which a reed valve assembly 214 is installed. A secondary opening 216, which couples to opening 206 associated with the secondary intake ports, is not occluded.

In FIG. 11, a cross section perpendicular to the axis of the cylinder bore taken through the primary intake ports is shown. The cross section shown in FIG. 11 includes frame 210, intake ducts 220, and intake ports 226. The embodiment in FIG. 11 has intake ducts 220 and reed valve assemblies 214 located on two sides of block 200. The walls separating ports 226 are provided at an angle so that they induce a swirl flow.

In FIG. 12, a cross section taken along the axis of the cylinder bore is shown. Liner 222 has multiple exhaust ports 224, primary intake ports 226, and secondary intake ports 228. When the reed valves are open, gases flow from intake duct 220 through primary intake ports 226, and into the cylinder defined by liner 222. Flow from intake duct 220 goes through secondary intake ports 228 into the cylinder defined by liner 222 unimpeded. The piston that uncovers the intake ports is not shown in FIG. 12. However, referring back to FIGS. 3, 5, and 7, it can be seen that the piston uncovers the primary ports first and later in the expansion stroke uncovers the secondary ports. Thus, the reed valves prevent backflow of exhaust gases into intake duct 220 when it is most likely, i.e., when pressure in the cylinder is higher. By the time that secondary ports 228 are uncovered, the pressure in the cylinder is lower and thus reed valves are not provided in the duct leading to secondary intake ports 228.

The embodiment illustrated in FIGS. 9-12 with reed valves in the duct associated with primary intake ports but no reed valves in secondary intake ports 228 is a combination that provides minimal pressure drop while preventing backflow during the portion of the expansion stroke in which primary intake ports 226 are uncovered. It has been found that with such a configuration, the appropriate asymmetry is about 15 degrees, thereby significantly reducing the amount of piston imbalance compared with a 20 degree asymmetry. Five degrees of asymmetry from absolute symmetry causes a minimal imbalance while five degrees, e.g., from 15 to 20 degrees has much more impact on imbalance.

In FIG. 13, port open area is plotted as a function of crank angle degree. Dashed line 300 indicates the area uncovered by an exhaust piston as it reciprocates in the cylinder. The port open area for the primary intake ports is plotted as a dash-dot line 302. The intake piston completely uncovers the primary intake ports at about 137 degrees crank angle. The open area of the primary intake ports, as shown by line 302, does not increase further, i.e., remains constant from about 137-242 degrees crank angle. The beginning of the opening of the secondary intake ports doesn't occur until after the primary intake ports are completely uncovered. There is a slight additional delay due to there being a bridge between the primary and secondary intake ports. In the example shown in FIG. 13, the dash-dot-dot line 3304 indicating the port open area for the secondary ports has a flat portion in the range of 180 to 200 degrees crank angle, which indicates that the secondary intake ports are completely open during this range of intake piston movement. The total intake port open area, i.e., sum of the primary and secondary intake ports, is shown as solid line 306. The shoulders 308 coincide with the piston movement associated over the bridge area between the primary and secondary intake ports.

The port timings and areas that are shown in FIG. 13 have a number of unfavorable characteristics. It is desirable for there to be about a 30 crank angle degree delay between the opening of the exhaust ports and the intake ports to allow exhaust blowdown to occur so that the pressure in the cylinder is greatly reduced and so that the blowdown energy is directed into the exhaust for recovery in an exhaust turbine or other recovery device. The primary exhaust ports, however, begin to open at about 10 degrees crank angle after the exhaust ports begin to open. Also, it is desirable for the intake ports to be closed not too late into the compression stroke. In FIG. 13, the intake ports close at about 260 degrees crank angle. With piston porting, about the only way to reduce the duration of intake port opening is to reduce the height of the intake ports. If such a measure were taken, then not only would the duration of the intake port open be reduced, but the maximum port open area, curve 306, is significantly reduced. It is desirable for the intake opening area to be greater than exhaust opening area. Exhaust gases flow out of the cylinder under high pressure in the cylinder, i.e., a great pressure difference driving the flow of exhaust gases out of the cylinder. On the other hand, intake gases are driven by a lesser pressure difference and thus a higher flow area is desired to facilitate the induction of fresh intake gases.

According to an embodiment of the disclosure, reed valves are placed upstream of the primary intake ports. Thus, although the intake ports may be uncovered by the intake piston, flow through the intake ports is prevented by the reed valves. The effect of the reed valves on the effective intake port open area is shown by dotted line 310 (in regards to opening of the reed valves) and short dash-long dash line 312 (in regards to closing of the reed valves). The reed valves allow for a large port open area, but with a desirable open duration. The reed valve opening 310 and closing 312 are one example. Reed valves open and close based on the relative pressures on the upstream and downstream sides of the reed valve and the flow conditions past the reed valves. Thus, the opening 310 and the closing 312 of the reed valves as depicted in FIG. 13 are but one example. The actual opening and closing of the reed valves may vary somewhat from the example shown in FIG. 13.

While the best mode has been described in detail with respect to particular embodiments, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. While various embodiments may have been described as providing advantages or being preferred over other embodiments with respect to one or more desired characteristics, as one skilled in the art is aware, one or more characteristics may be compromised to achieve desired system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments described herein that are characterized as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications. 

We claim:
 1. An internal combustion engine, comprising: a cylinder; a crankshaft; a piston reciprocating in the cylinder with the piston coupled to the crankshaft via a rod; a first plurality of intake ports defined in the cylinder at a first predetermined distance from an axis of rotation of the crankshaft; a second plurality of intake ports defined in the cylinder at a second predetermined distance from the axis of rotation of the crankshaft wherein the first and second pluralities of ports fluidly couple between a combustion chamber defined by a top of the piston and the cylinder; a primary intake duct coupled to the first plurality of intake ports with a reed valve disposed in the primary intake duct adjacent to the intake ports; and a secondary intake duct coupled to the second plurality of intake ports.
 2. The engine of claim 1 wherein the first and second pluralities of intake ports are substantially evenly disposed around a periphery of the cylinder.
 3. The engine of claim 1, further comprising: normally-closed, unidirectional flow valves disposed upstream of the first plurality of intake ports wherein the unidirectional valves open when pressure on the upstream side of the unidirectional valves exceeds pressure on the downstream side of the unidirectional valves.
 4. The engine of claim 3 wherein the normally-closed, unidirectional valves comprise a reed valve block having a frame with a plurality of openings, flexible petals adapted to cover the openings, and stops that limit the movement of the petals.
 5. The engine of claim 1 wherein the first plurality of intake ports and the second plurality of intake ports do not overlap in a direction parallel to a motion of the piston in the cylinder.
 6. The engine of claim 1, further comprising: a high-pressure compressor fluidly coupled to the first plurality of intake ports; and a low-pressure compressor fluidly coupled to the second plurality of intake ports.
 7. The engine of claim 6 wherein the second plurality of intake ports is closer to the crankshaft than the first plurality of intake ports.
 8. The engine of claim 1 wherein the piston is an intake piston, the engine further comprising: a first plurality of exhaust ports defined in the cylinder at a third predetermined distance from the axis of rotation of the crankshaft; and an exhaust piston reciprocating in the cylinder with a top of the intake piston facing a top of the exhaust piston in an opposed-piston arrangement wherein the intake piston covers and uncovers the first and second pluralities of intake ports when the intake piston reciprocates in the cylinder and the exhaust piston covers and uncovers the first plurality of exhaust ports when the exhaust piston reciprocates in the cylinder.
 9. The engine of claim 8, further comprising: a second plurality of exhaust ports defined in the cylinder at a fourth predetermined distance from the axis of rotation of the crankshaft; a high-pressure compressor coupled to the first plurality of intake ports; a low-pressure compressor coupled to the second plurality of intake ports; a high-pressure turbine fluidly coupled to the first plurality of exhaust ports; and a low-pressure turbine fluidly coupled to the second plurality of exhaust ports wherein the high-pressure compressor is shaft-coupled to the high-pressure turbine and the low-pressure compressor is shaft-coupled to the low pressure turbine.
 10. The engine of claim 1 wherein no valve is disposed in the secondary intake duct.
 11. An internal combustion engine, comprising: a cylinder; a crankshaft; an intake piston reciprocating in the cylinder with the intake piston coupled to the crankshaft via a rod; an exhaust piston disposed in the cylinder in an opposed-piston arrangement with respect to the intake piston; a plurality of primary intake ports defined in the cylinder at a first predetermined distance from an axis of rotation of the crankshaft; a plurality of exhaust ports defined in the cylinder at a third predetermined distance from the axis of rotation of the crankshaft; a plurality of secondary intake ports defined in the cylinder at a second predetermined distance from the axis of rotation of the crankshaft; a primary intake duct coupled to the primary intake ports; normally-closed, unidirectional valves disposed in the primary intake duct; and a secondary intake duct coupled to the secondary intake ports wherein the intake piston uncovers the intake ports during an expansion stroke of the intake piston and the exhaust piston uncovers the exhaust ports during an expansion stroke of the exhaust piston.
 12. The engine of claim 11, further comprising: reed valves disposed in the secondary intake duct.
 13. The engine of claim 11 wherein the secondary intake duct has no valves disposed therein.
 14. The engine of claim 11 wherein the unidirectional valves comprise a reed valve block.
 15. The engine of claim 11 wherein the primary intake duct is fluidly decoupled from the secondary intake duct at least for a portion of the lengths of the first and secondary intake ducts on an end of the intake ducts proximate the intake ports.
 16. An internal combustion engine, comprising: a cylinder having a plurality of primary intake ports defined in the cylinder at a first predetermined distance from a first end of the cylinder and a plurality of secondary intake ports defined in the cylinder at a second predetermined distance from the first end of the cylinder; a primary intake duct coupled to the primary intake ports; a secondary intake duct coupled to the secondary intake ports wherein the primary intake duct is fluidly decoupled from the secondary intake duct at least for a portion of the lengths of the first and secondary intake ducts proximate the intake ports; an intake plenum fluidly coupled to the primary and secondary intake ducts; and a unidirectional valve disposed in the primary intake duct located between the intake plenum and the primary intake ports adjacent to the primary intake ports.
 17. The engine of claim 16 wherein the primary and second intake ports fluidly coupled with a combustion chamber defined by a top of the piston and the cylinder.
 18. The engine of claim 16, further comprising: a reed valve block disposed in the primary intake duct.
 19. The engine of claim 16 wherein the reed valve block comprises: a frame having openings and flexible reed petals adapted to occlude the openings.
 20. The engine of claim 16, further comprising: a high-pressure compressor fluidly coupled to the primary intake duct; and a low-pressure compressor fluidly coupled to the second intake duct. 